1. Field of the Invention
The present claimed invention relates to the field of semiconductor device fabrication. More specifically, the present claimed invention relates to the deposition of photoresist onto semiconductor wafers.
2. Prior Art
During conventional applications of photoresist coatings to semiconductor wafers, a "coater" system is used. One part of the coater system is a fiat, circular, disk-shaped, rotating vacuum chuck having a diameter slightly less than that of a semiconductor wafer. The vacuum chuck is used to hold and rotate a semiconductor wafer during the photoresist application process. The vacuum chuck is oriented such that a semiconductor wafer placed thereon resides in a level horizontal plane. In operation, the bottom or inactive surface of a semiconductor wafer is placed onto the vacuum chuck. The vacuum chuck applies a suction or negative pressure to the bottom surface of the semiconductor wafer to hold the semiconductor wafer on the vacuum chuck. In standard photoresist coater systems, an axle extends downward from the vacuum chuck and is powered by motors to rotate the vacuum chuck.
Commonly, a desired amount of liquid photoresist is applied to the top upwardly-facing surface of the semiconductor wafer while the semiconductor wafer is being rotated on the vacuum chuck. Thus, as the semiconductor wafer is rotating, the photoresist material spreads radially outward from the center of the semiconductor wafer towards the edge of the semiconductor wafer such that the entire top or active surface of the wafer is coated with a layer of photoresist. Excess photoresist material is sloughed off of the wafer during the rotation process. The fixed speed of rotation of the semiconductor wafer and the amount of photoresist applied are set at different fixed values in an attempt to achieve a layer of photoresist of a uniform desired or "target" thickness.
Unfortunately, variations from the target thickness of the photoresist layer commonly occur across the surface of the semiconductor wafer. That is, for example, the thickness of the photoresist layer may be much greater at the center of the wafer than at the edge of the wafer. Other variations are also possible such as, for example, a "bowl-shaped" distribution wherein the photoresist layer is much thicker at the edges of the wafer than at the center thereof.
In addition to variations in the thickness of the photoresist layer across the surface of a single semiconductor wafer, unwanted variations also occur from wafer to wafer. That is, a first wafer may have a first mean photoresist layer thickness, and a second wafer processed concurrently with the first wafer will have a second mean photoresist layer thickness wherein the first and second mean thicknesses are different from each other.
It is well known that variations from the target thickness of the photoresist layer across the surface of the semiconductor wafer deleteriously affect critical dimensions during subsequent process steps which the semiconductor wafer undergoes. Specifically, variations in the photoresist layer from the target thickness affect, for example, linewidth control at the poly layer. That is, at the target thickness a certain linewidth will be obtained during subsequent photolithography steps. However, different linewidths will be generated on the same wafer if the thickness of the photoresist layer varies from the target thickness. Thus, the uniformity of the photoresist layer is critical to achieving, for example, precise linewidth control at the poly layer.
In an attempt to achieve a uniform layer of photoresist material across the top surface of a semiconductor wafer, a large volume of photoresist material may be applied to the top surface of the wafer. While the wafer is spinning at a high rate of speed, the photoresist material coats the surface of the wafer and the excess photoresist material is cast off of the wafer as waste. However, due to the extremely high cost of photoresist material, such practices dramatically increase process costs. Therefore, in an attempt to reduce the amount of photoresist required, the fixed wafer spin speed is set at a higher fixed rate, and/or the dispensed amount of photoresist is decreased. However, the reduced amount of photoresist and the increased fixed wafer spin speed can result in incomplete coverage and even in less uniformity in the thickness of the layer of photoresist across the surface of the semiconductor wafer.
Commonly, some loss of uniformity in the layer of photoresist material across the top surface of the semiconductor wafer has been tolerated in exchange for the cost savings which are realized by using less photoresist material. That is, even though the increased wafer speed and reduced amount of photoresist result in variations from the target thickness of the photoresist layer, in the past such adverse results have been tolerated in order to reduce process costs.
Thus, the need has arisen for a method or process which simultaneously provides for the formation of a uniform layer of photoresist material onto the top surface of a semiconductor wafer and which does so using a decreased amount of photoresist material.